The objective of the proposed research is to develop a sufficient understanding of the snail-trematode and intertrematode interactions to be able to utilize this approach as a means for developing biological control methods. Emphasis will be given to the study of acquired resistance to trematode infections in snails, a newly discovered phenomenon that should open new possibilities in the study of biocontrol of trematode diseases. Of particular interest is also the observation that living sporocysts have the capability of actively inhibiting or interfering with the snail's defense. With the use of appropriate snail-trematode systems, appropriate combinations of irradiated and nonirradiated trematode species, controlled dosage and infection intervals, along with standard histological and EM techniques, we shall study the underlying principles of acquired resistance. This includes the study of the amebocyte-producing organ which enlarges considerably during sensitization of the snail to regress again after the sensitization process is completed; the study of whether humoral factors play a role in the snail's defense; the study of the specificity of natural resistance in juvenile and adult snails and the mechanism by which living sporocysts interfere with the snail's defense.